Herrings put into a tub with salt or brine, later spitted on sharp wooden spits, and hung up in a chimney to be smoked and dried. Such preserved fish would keep for months (and indeed they were transported in barrels to provide protein on long sea voyages) but were inedible in this state and needed to be soaked to soften them and remove the salt before they could be cooked.

Do I smell a rat?

One key characteristic of red herrings, apart from their colour, was their strong smell, so much so that one use for them on occasion was to train hounds to follow a scent.

In novels, usually mystery novels, a "red herring" is an extraneous character meant to divert the reader's attention from the true killer/robber/etc. It is usually a logical choice for the culprit, but ends up being nothing.

I thought Hampton was the killer, but he was a total red herring. It was Muldoon.

A term used to describe a guy/girl who you're going out with/fucking just to get over someone else you can't get.

Red herrings were often used to mislead hound dogs; the expression refers to something that is misleading.

Therefore - you try to convince yourself that you like someone (to mislead you/lead you away from the person you actually like), and call him/her your 'red herring'.

A: I'm so in love with him but he's dating someone :(
B: I'm so sorry :( what are you going to do?
A: I think I might go for Tony.
B: Tony? Why! Do you even like him?!
A: Nah, but he'll just be my red herring.